Effect of High Effective Stress on the Permeability of Shale Rock Undergoing Shear Deformation Fracturing: A Comprehensive Study

Understanding how permeability changes in response to shear displacement is vital in the development of shale gas and other sources of unconventional gas or oil. In previous studies, experiments on the dependence of the permeability of shale on confining stress were mostly carried out under low-stress conditions (<40 MPa). Hence, the relationship between shear rock fracturing in the presence of high effective stress and the mechanism responsible for the permeability change that occurs remains unclear. In this study, numerical models were established, and laboratory experiments were conducted over the 1.5-59.5 MPa range of effective confining stress to unveil the dynamic permeability changes occurring in shale with rough fracture surfaces. The changes in the morphology of the internal fractures induced by different shear deformations were also revealed by measuring the joint roughness coefficient of the fracture surfaces and applying computed tomography after each set of experiments. Finite-element numerical models were generated (using COMSOL Multiphysics 5.4) by embedding randomly generated rough surfaces inside cylindrical-shaped shale rock samples. These were subsequently used to investigate the mechanism underlying the change in permeability. The effect of high effective stress on the change that occurs in the fracture permeability during shear deformation was also analyzed, along with the effective flow area and equivalent fracture aperture. Under conditions where shear deformation is 1 mm, the self-supporting mechanism is dominant and favors the establishment of equivalent flow fracture apertures and increase in permeability. The effect of overlapping becomes more important as the shear deformation increases, and this decreases the effective flow area and hence the permeability. Under high effective stress, the overlapping mechanism also causes the fracture surfaces to become damaged, which inevitably generates wear products in the fractures and reduces the permeability. This work provides new insights into the permeability evolution of rock strata under shear action.

Automated summary

Understanding the permeability changes in shale under shear displacement is crucial for the development of unconventional gas or oil sources. Previous studies mainly focused on experiments under low-stress conditions, leaving the mechanism behind the permeability change in the presence of high effective stress unclear. This study used numerical models and laboratory experiments to investigate the dynamic permeability changes in shale with rough fracture surfaces. The morphology of internal fractures induced by different shear deformations was also examined. The results provide new insights into the permeability evolution of rock strata under shear action.